The Bulgarians steam-roll the Byzantine Empire from the west, whilst another polity invades Anatolia.
The Byzantines manage to hold Anatolia and Constantinople, but lose the Balkans to the Bulgarians. They focus on reclaiming the Levant and the East. Success in the Levant and Egypt convinces the Emperor that relocating to Alexandria, or a ready-built capital may be worth it since Bulgaria is too strong after a successful invasion of Pannonia.
After a decade of transferring the wealth and people of Constantinople to Alexandria (Or alt-capital) the Byzantine Emperor institutes a plan that renders Constantinople a key trade port and military centre, rather than capital.
The Alexandrians (as historians would call this period) manage to survive rather well, resolving issues between Coptic and Orthodox Christianity, focusing on conquering Ethiopia.
The Alexandrians eventually go east, following the trade routes, setting up colonies, whilst the west eventually circumnavigates Africa to avoid paying Alexandrian prices for goods.
Meanwhile, the Bulgarians maintain control over the Balkans, eventually displacing the Greek population, which centres in Anatolia.
By modern times, the Alexandrian Empire has lost Anatolia (known as Greece), and many of its colonial possessions (although it is still a trade power) - Bulgaria eventually took Constantinople and has its capital in Tsargrad.